为什么 1000==1000 返回为false，而 100==100 会返回为true

现象

Integer a = 1000, b = 1000;
System.out.println(a == b);//false
Integer c = 100, d = 100;
System.out.println(c == d);//true

分析

== 运算比较的不是值而是引用

当我们给一个Integer 对象赋一个int值的时候，会调用Integer类的静态方法valueOf，如果看看valueOf 的源代码就知道发生了什么：

public static Integer valueOf(int i) {
  if (i >= IntegerCache.low && i
	  return IntegerCache.cache[i + (-IntegerCache.low)];
  return new Integer(i);
}

IntegerCache 是Integer 的内部类，其代码如下所示：

/**
     * Cache to support the object identity semantics of autoboxing for values between
     * -128 and 127 (inclusive) as required by JLS.
     *
     * The cache is initialized on first usage.  The size of the cache
     * may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
     * During VM initialization, java.lang.Integer.IntegerCache.high property
     * may be set and saved in the private system properties in the
     * sun.misc.VM class.
     */

    private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];

        static {
            // high value may be configured by property
            int h = 127;
            String integerCacheHighPropValue =
                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                try {
                    int i = parseInt(integerCacheHighPropValue);
                    i = Math.max(i, 127);
                    // Maximum array size is Integer.MAX_VALUE
                    h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            }
            high = h;

            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);

            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }

        private IntegerCache() {}
    }

简单的说，如果整型字面量的值在-128 ~ 127之间，那么不会new新的Integer对象，而是直接引用常量池中的Integer对象，所以才会出现上面的现象